The present invention relates to organic electroluminescent devices and more particularly to doping an emission layer and a hole-transport layer and/or an electron-transport layer with suitable color-neutral dopants to improve the operational lifetime of these devices without affecting the electroluminescent color.
A common form of organic electroluminescent device (OLED) includes a substrate and, disposed thereupon, a multilayer structure comprising an anode, an optional hole-injection layer, a hole-transport layer, an emission layer, an optional electron-transport layer, and a cathode, each layer of the multilayer structure comprising one or more organic or organometallic compounds. Electroluminescent (EL) devices are attractive because of their low driving voltage, high luminance, wide viewing angle, and capability for full-color flat-panel emissive displays. Tang et al. described this OLED structure in their U.S. Pat. Nos. 4,769,292 and 4,885,211. With the prospect of using OLED in display screens for laptop computers, digital personal organizers, cellular telephones etc., it is highly desirable to improve the operational lifetime of these devices.
The following patents and publications disclose the preparation of OLED with improved operational lifetime. Modifications of the multilayer structure, stabilized cathode materials, and confinement of various carriers and their recombination within the emission zone have achieved significant improvement in the operational lifetime of these devices. So et al., in U.S. Pat. No. 5,853,905, discussed an EL device consisting of a single organic emission layer containing a mixture of electron-transport and hole-transport materials, sandwiched between anode and cathode. However, this device has low efficiency. Popovic et al., in SPIE Conference Proceedings, Vol. 3476, pp. 68-72, 1998, described an EL device with improved efficiency and operational lifetime prepared by mixing an emitting electron-transport material and a hole-transport material in a light-emitting layer. Xie et al., in U.S. Pat. No. 5,989,737, disclosed an OLED in which the hole-transport layer comprises a tertiary aromatic amine doped with a polycyclic aromatic hydrocarbon such as rubrene. This approach has the problem that some dopants, such as rubrene, alter the electroluminescent color of the device.
The color of the EL emission can be altered by employing a binary mixture of materials in the emission layer. The major component of the mixture, known as the host material, is responsible for transport of electrical current, and the minor component, known as the dopant, is responsible for the EL emission. For this use, the dopant is usually a luminescent dye, and the color of the emission is characteristic of the dopant. A dopant can also be used to increase the EL efficiency without substantially changing the EL color. Takeuchi et al., Jpn. Journal of Applied Physics, Vol. 31, Part 2, No. 4B, pp. L 498-L500, 1992 and Japanese Patent# JP11273861-A disclose the use of anthracene as such a dopant and tris(8-quinolinolato-N1,O8)aluminum, also known as 8-hydroxyquinoline aluminum, as the host.
It is therefore an object of the present invention to provide an organic light-emitting device comprising:
a) a substrate;
b) an anode and a cathode positioned relative to the substrate;
c) an emission layer disposed between the anode and the cathode;
d) a hole-transport layer disposed between the anode and the emission layer;
e) an electron-transport layer disposed between the cathode and the emission layer;
f) the hole-transport layer including one or more sublayers, the sublayer closest to the emission layer containing a color-neutral dopant;
g) the emission layer containing a color neutral dopant; and
h) the color-neutral dopant including an anthracene derivative.
A color-neutral dopant is a dopant that does not substantially affect the electroluminescent color of the device relative to that of an otherwise identical device from which the dopant is omitted.
The use of a color-neutral dopant permits the electroluminescent color of the present OLEDs to be tuned solely by adjusting the composition of the emission layer. It has been found that certain anthracene derivatives can be used as color-neutral dopants in the emission layer and in the hole-transport and/or the electron-transport layer of an OLED and that the resulting device has a greater operational lifetime than an otherwise identical device that does not contain the color-neutral dopant.
Other features and advantages of the present invention include the following:
OLEDs with various electroluminescent colors, such as red, green, blue, or white, can be constructed by inclusion of suitable luminescent dopants in the emission layer. Thereby, it is possible to construct OLEDs with simultaneously optimized lifetime and chromaticity.
OLEDs of this invention have high luminance efficiency.
OLEDs of this invention can be used with a low-voltage current source.
Inclusion of a color-neutral dopant in the emission layer and simultaneously in the hole-transport layer and/or the electron-transport layer provides an OLED with an operational lifetime greater than that of an OLED including the color-neutral dopants in only the hole-transport layer and/or the electron-transport layer but not the emission layer.
The devices made in accordance with the present invention have long operational lifetimes and excellent luminance properties.